Medical institutions are equipped with radiographic apparatus 51 for obtaining a tomographic image of a subject M. Such radiographic apparatus 51 includes a configuration in which a radiation source 53 that emits radiation and an FPD 54 that detects radiation move synchronously to continuously take a series of fluoroscopic images, and then the series of fluoroscopic images are superimposed to obtain the sectional image (see FIG. 13.) In such radiographic apparatus 51, during taking a series of fluoroscopic images, the radiation source 53 and the FPD 54 move along a body axis direction A of the subject M as to approach to each other, thereby having the same position in the body axis direction A. Thereafter, the radiation source 53 and the FPD 54 move along the body axis direction A as to be spaced away from each other. Such radiographic apparatus is described, for example, in Patent Literature 1.
Description will be given of operation in taking the sectional image as above. Firstly, the radiation source 53 intermittently emits radiation while moving. Specifically, the radiation source 53 moves along the body axis direction of the subject M for every completion of irradiation, and again emits radiation. In this way, 74 fluoroscopic images are obtained, and then superimposed. The finished image is a tomographic image having a sectional image appearing when the subject is cut along a sectional plane.
The radiographic apparatus 51 may also take a radioscopic image that simply visualizes the subject M through various settings. For obtaining such radioscopic image in this way, the radiation source 53 and the FPD 4 each move into a given position. During this movement, a fluoroscopic image is once taken while the subject M is placed. Such radiography is referred to as spot radiography as distinction from tomography.
Next, description will be given of how the FPD 4 detects radiation emitted from the radiation source 53. Radiation entering the FPD 4 is once converted into an electric signal and amplified by a given magnification. Thereafter, the electric signal is converted into a digital signal. This amplification factor corresponds to an analog gain. It is necessary to irradiate a subject with a certain dose of radiation for obtaining a radioscopic image suitable for diagnosis. It is assumed that an extremely low dose of radiation is applied for suppressing exposure radiation to the subject. In this case, as an analog gain increases by a decreased dose of radiation, a noise component contained in the electric signal is also amplified. Accordingly, a granular false image is to be contained in the radioscopic image acquired. On the other hand, the radioscopic image has a reduced contrast, since an electric signal is not sufficiently amplified unless an analog gain increases. As noted above, the conventional configuration is set to have an analog gain not more than a certain value and a dose of radiation not less than a certain value in order to prevent reduction in visibility of the radioscopic image.    [Patent Literature 1] Japanese Patent Publication No. 2006-271513